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OMI UV spectral irradiance: comparison with ground based measurements in an urban environment. Stelios Kazadzis A. Bais, A. Arola OMI science team meeting Helsinki, June 2008 Finnish Meteorological Institute Laboratory of Atmospheric Physics, Thessaloniki, Greece. Outline. - PowerPoint PPT Presentation
Citation preview
21.04.23
OMI UV spectral irradiance: comparison with ground based measurements in an urban environment
Stelios KazadzisA. Bais, A. Arola
OMI science team meeting Helsinki, June 2008
Finnish Meteorological InstituteLaboratory of Atmospheric Physics, Thessaloniki, Greece
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 2
Outline
• OMI – ground based UV spectral irradiance comparison – statistics
• Aerosol absorption – post correction approaches
• Campaign: Spacial and temporal UV variability within an OMI grid
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 3
• The problem - absorbing aerosolsCurrent OMI UV algorithm does not account for absorbing aerosols
(e.g. organic carbon, smoke, dust )
OMI – GB UV comparison – statistics
Tanskannen et al., JGR 2007
Tokyo: +32%
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 4
Thessaloniki Area
OMI – GB UV comparison – statistics
forest
agricu lture
w etlands
grasslands
other
Thessaloniki
Longitude (degrees)
Latit
ude
(deg
rees
)
-High aerosol load
- Aerosol transport Sahara dust intrusions Biomass burning from NE
- Very high PM10 conc.
Sahara - Dream model
Fire hot spots - summer
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 5
Instrumentation - comparisonOMI – GB UV comparison – statistics
Rooftop of the School of Natural Sciences
Brewer instrumentBrewer instrument(spectral-calibrated, wavelength shift corrected)(spectral-calibrated, wavelength shift corrected)
UV irradiance at 305, 324, 380 nm and CIEUV irradiance at 305, 324, 380 nm and CIETotal column ozoneTotal column ozonespectral AOD at UV wavelengths spectral AOD at UV wavelengths
CIMELCIMELAOD (340nm) , SSA(440nm), ..AOD (340nm) , SSA(440nm), ..
NILU-UV NILU-UV 305nm, 324nm, 340nm, 380nm305nm, 324nm, 340nm, 380nm
Cloud – cloudless case separationCloud – cloudless case separationPyranometer, observations, sky cameraPyranometer, observations, sky camera pix pix
Daily OMI overpass time (mean over Daily OMI overpass time (mean over ±±15 minutes)15 minutes)
Thessaloniki September 2004-December 2007Thessaloniki September 2004-December 2007
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 6
Results
0 20 40 60 80OMI
0
20
40
60
80
Bre
wer
305 nm
0 100 200 300 400 500OMI
0
100
200
300
400
500
Bre
wer
324 nm
OMI – GB UV comparison – statistics
305nm OMI +30% 324nm OMI +17%
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 7
Results
0 1 2 3OMI - GB ratio
0.0
0.1
0.2
0.3
ΔΝ
/Ν
380 nm
Days
cloud free
cloudy
OMI – GB UV comparison – statistics
0 1 2 3OMI - GB ratio
0.0
0.1
0.2
0.3
ΔΝ
/Ν
C IED
Days
cloud free
cloudy
380nm OMI +11% CIED OMI +20%
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 8
Results - statistics
Wavel.(nm)
All data cloudless
m R2 W10 W20 m R2 W10 W20
305 1.30 0.94 43.3 68.2 1.27 0.95 64.8 87.6
324 1.17 0.89 51.3 73.1 1.15 0.91 76.9 92.4
380 1.13 0.89 47.2 70.7 1.11 0.91 71.8 89.7
CIED 1.20 0.93 51.5 75.4 1.19 0.95 75.0 91.9
OMI – GB UV comparison – statistics
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 9
Results - statistics
Wavel.(nm)
All data cloudless
m R2 W10 W20 m R2 W10 W20
305 1.30 0.94 43.3 68.2 1.27 0.95 64.8 87.6
324 1.17 0.89 51.3 73.1 1.15 0.91 76.9 92.4
380 1.13 0.89 47.2 70.7 1.11 0.91 71.8 89.7
CIED 1.20 0.93 51.5 75.4 1.19 0.95 75.0 91.9
OMI – GB UV comparison – statistics
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 10
Post correction methods
Krotkov et al., OE 2004Krotkov et al., OE 2004
)(1
)()(
abs
ac b
EE
Post correction methods - TOMS experience
Arola et al., JGR 2005
Cloudless cases: Ta(Cloudless cases: Ta(λλ) = AOD() = AOD(λλ) * [1 - SSA() * [1 - SSA(λλ)])]
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 11
UV attenuation – Thessaloniki, cloudless casesPost correction methods
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonths
-35
-30
-25
-20
-15
-10
-5
0
% U
V a
tten
uat
ion
du
e to
aer
oso
l at
(324
nm
)
UV attenuation OMI-Ground based % bias
77% of cases
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 12
Post correction : method 1
0.00 0.02 0.04 0.06 0.08 0.10 0.12Absorption optical depth
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Irra
dia
nce
rat
io O
MI /
Bre
wer
at
324
nm
Slope = 1.5 per unit of absorption optical depthIntercept = 1.10
ssa >0.98
ssa= [0.94, 0.98]
ssa= [0.88, 0.94]
ssa < 0.88
Ta (Ta (λλ) = AOD() = AOD(λλ) * [1 - SSA(440nm)]) * [1 - SSA(440nm)]
Aerosol absorption CFAerosol absorption CF((λλ)) = 1.1 + 1.5 * Ta = 1.1 + 1.5 * Ta((λλ))
Post correction methods
•no sza dependenceno sza dependence•SSA @ UV ?SSA @ UV ?•need of GB data need of GB data
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 13
Post correction: method 2Tas =Ta / cos(sza)
Aerosol absorption CFAerosol absorption CF((λλ)) = 1.07 + 1.8 * Tas = 1.07 + 1.8 * Tas((λλ))
Post correction methods
•SSA @ UV ?SSA @ UV ?•need of GB data need of GB data
0.00 0.04 0.08 0.12slant column absorption optical depth
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Irra
dia
nce
rat
io O
MI /
Bre
wer
at
324
nm
Slope = 1.8 per unit of slant absorption optical depthIntercept = 1.07
ssa >0.98
ssa= [0.94, 0.98]
ssa= [0.88, 0.94]
ssa < 0.88
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 14
Post correction: use of RT model
0 0.2 0.4 0.6 0.8 1 1.2 1.41
1.1
1.2
1.3
1.4
1.5
1.6
1.7
aerosol optical depth at 340 nm
corr
ecti
on
fo
r ir
rad
ian
ce a
t 32
4nm
sza=40
0.80.820.840.860.88 0.90.920.940.960.98 1
0.4 0.45 0.5 0.55 0.61.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
1.2
aerosol optical depth at 340 nm
corr
ecti
on
fo
r ir
rad
ian
ce a
t 32
4nm
sza=60
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
ssa=0.88
ssa=1, no absorption
total correction
correction for scattering
Post correction methods
S1: AOD and SSA synchronous measurementsS2: AOD and SSA@440 = constS3: AOD= const and SSA@340 = const
Abs + scat
scat
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 15
Overview of post corrections
6th Approach: CF(λ) = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
OMI/Brewer ratio [R]
Method 305 nm Mean (1σ)
324 nm Mean (1σ)
380 nm Mean (1σ)
Obs
Original No correction 1.27 (0.15) 1.15 (0.10) 1.11 (0.12) 267
S1 Apply Ta slope 1.17 (0.13) 1.07 (0.09) 1.05 (0.13) 135
S2 Apply Tas slope 1.18 (0.13) 1.09 (0.10) 1.05 (0.13) 135
M1 Model 1.13 (0.12) 1.04 (0.08) 1.01 (0.11) 135
M2 Model constant SSA 1.14 (0.13) 1.03 (0.09) 0.99 (0.10) 267
M3 Model const. SSA-AOD 1.12 (0.13) 1.02 (0.09) 0.98 (0.10) 267
S3 1 + 3 * Ta (λ) 1.11 (0.13) 1.03 (0.10) 1.01 (0.11) 135
Post correction methods
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 16
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
OMI/Brewer ratio [R]
Method 305 nm Mean (1σ)
324 nm Mean (1σ)
380 nm Mean (1σ)
Obs
Original No correction 1.27 (0.15) 1.15 (0.10) 1.11 (0.12) 267
S1 Apply Ta slope 1.17 (0.13) 1.07 (0.09) 1.05 (0.13) 135
S2 Apply Tas slope 1.18 (0.13) 1.09 (0.10) 1.05 (0.13) 135
M1 Model 1.13 (0.12) 1.04 (0.08) 1.01 (0.11) 135
M2 Model constant SSA 1.14 (0.13) 1.03 (0.09) 0.99 (0.10) 267
M3 Model const. SSA-AOD 1.12 (0.13) 1.02 (0.09) 0.98 (0.10) 267
S3 1 + 3 * Ta (λ) 1.11 (0.13) 1.03 (0.10) 1.01 (0.11) 135
Post correction methods
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 17
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
OMI/Brewer ratio [R]
Method 305 nm Mean (1σ)
324 nm Mean (1σ)
380 nm Mean (1σ)
Obs
Original No correction 1.27 (0.15) 1.15 (0.10) 1.11 (0.12) 267
S1 Apply Ta slope 1.17 (0.13) 1.07 (0.09) 1.05 (0.13) 135
S2 Apply Tas slope 1.18 (0.13) 1.09 (0.10) 1.05 (0.13) 135
M1 Model 1.13 (0.12) 1.04 (0.08) 1.01 (0.11) 135
M2 Model constant SSA 1.14 (0.13) 1.03 (0.09) 0.99 (0.10) 267
M3 Model const. SSA-AOD 1.12 (0.13) 1.02 (0.09) 0.98 (0.10) 267
S3 1 + 3 * Ta (λ) 1.11 (0.13) 1.03 (0.10) 1.01 (0.11) 135
Post correction methods
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 18
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
OMI/Brewer ratio [R]
Method 305 nm Mean (1σ)
324 nm Mean (1σ)
380 nm Mean (1σ)
Obs
Original No correction 1.27 (0.15) 1.15 (0.10) 1.11 (0.12) 267
S1 Apply Ta slope 1.17 (0.13) 1.07 (0.09) 1.05 (0.13) 135
S2 Apply Tas slope 1.18 (0.13) 1.09 (0.10) 1.05 (0.13) 135
M1 Model 1.13 (0.12) 1.04 (0.08) 1.01 (0.11) 135
M2 Model constant SSA 1.14 (0.13) 1.03 (0.09) 0.99 (0.10) 267
M3 Model const. SSA-AOD 1.12 (0.13) 1.02 (0.09) 0.98 (0.10) 267
S3 1 + 3 * Ta (λ) 1.11 (0.13) 1.03 (0.10) 1.01 (0.11) 135
Post correction methods
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 19
Correction results
0 10 20 30 40 50 60 70 800
10
20
30
40
50
60
70
80
OM
I ir
rad
ian
ce m
W/m
2n
m
Brewer irradiance mW/m2nm
305 nm
0 100 200 300 400 5000
100
200
300
400
500
OM
I ir
rad
ian
ce m
W/m
2n
m
Brewer irradiance mW/m2nm
324 nm
0 200 400 600 800 10000
200
400
600
800
1000
OM
I ir
rad
ian
ce m
W/m2n
m
Brewer irradiance mW/m2nm
380 nm
Post correction methods
305nm +11%305nm +11%
380nm +0%380nm +0%
324nm +2%324nm +2%
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 20
Effects of sza, AOD, SSA, ozone, time on ratios
10 20 30 40 50 60 70 800.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Sza (deg)
OM
I /
Bre
wer
rat
io a
t 32
4 n
m
original 1.15 (0.10)corr mod-c 1.03 (0.09)corr 1+1.5*Tabs 1.07 (0.09)corr 1+3*Tabs 1.03 (0.10)
Post correction methods
10 20 30 40 50 60 70 800.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
solar zenith angle (deg)
OM
I / B
rew
er r
atio
305
nm
original
corrected-mod
corrected mod-c
corrected 1+3*Tabs
0 0.02 0.04 0.06 0.08 0.1 0.120.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Tabs
OM
I /
Bre
wer
rat
io 3
80n
m
originalcorrected-modcorrected mod-ccorrected 1+3*Tabs
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 21
Campaign: 1 to 30 October, 2007Spatial and temporal UV variability within an OMI grid
•3 sites
Each:NILU UV at 305, 324, 380nmCIMEL (AOD, SSA, ..) Pyranometer, sky camera
Main site+ Brewers Spectral UV, ozoneCCD (spectral AOD)
2 Lidars (City – Rural)
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 22
UV Measurements at the three sites
02-Oct 07-Oct 12-Oct 17-Oct 22-Oct 27-Oct 01-Nov0
50
100
150
200
250
300
350
Day of 2007
Irra
dia
nce
at
324
nm
(m
W m
-2 n
m-1
)
NILU-UV AUTHNILU-UV EPANOMINILU-UV SINDOSOMIBrewer cloudlessBrewer all
Spatial and temporal UV variability within an OMI grid
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 23
AOD variability in an OMI grid
30-Sep 3-Oct 6-Oct 9-Oct 12-Oct 15-Oct 18-Oct 21-Oct 24-Oct 27-Oct 30-Oct
Day of year - 2007
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
AO
D a
t 34
0 n
m
RAIN
Location
Epanomi / rural site
Sindos / industrial site
AUTH / urban site
Spatial and temporal UV variability within an OMI grid
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 24
UV differences in an OMI grid
14-Oct 15-Oct 16-Oct 17-Oct 18-Oct 19-Oct
Day of year 2007
0.0
0.1
0.2
0.3
0.4
0.5
0.6
AO
D a
t 34
0 n
m
AUTH
BREW ER AUTH
SINDOS
-20
0
20
40
% U
V Irrad
iance d
ifference
from
AU
TH
(380 nm
)
EPANOMI
Spatial and temporal UV variability within an OMI grid
+20%+20%
-20%-20%
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 25
Spatial UV variability at 3 stations (2 * sigma / mean)*100
Spatial and temporal UV variability within an OMI grid
270 275 280 285 290 295 300 3050
20
40
60
80
100
120
Days of 2007
10
0*[
2*S
D /
me
an
]
UV Variability at 324 nm
10
20
30
40
50
60
Integration Time
90% of cloudless cases
80% of cloudy cases
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 26
Temporal UV variability
270 275 280 285 290 295 300 3050
20
40
60
80
100
120
Days of 2007
10
0*[
2*S
D /
me
an
]
UV Variability at 324 nm
10
20
30
40
50
60
Integration Time
90% of cloudless cases
80% of cloudy cases
0 10 20 30 40 50 6020
21
22
23
24
25
26
27
28
29
30
Integration Time (min)
10
0*
[ 2
*SD
/ m
ea
n]
Average UV variability of October 2007 for OMi overpass time
324 nm
Spatial and temporal UV variability within an OMI grid
(2 * sigma / mean)*100
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 27
Conclusions•3.5 years of OMI and ground based at Thessaloniki, Greece: measurement comparison3.5 years of OMI and ground based at Thessaloniki, Greece: measurement comparison showed an OMI overestimation of UV irradiances. showed an OMI overestimation of UV irradiances. Cloudless cases: Main reason is the aerosol absorption. Cloudless cases: Main reason is the aerosol absorption. Higher deviations at lower Higher deviations at lower wavelengthswavelengths
•Possible methods to correct this effect: AOD and SSA measurements or/and an aerosol Possible methods to correct this effect: AOD and SSA measurements or/and an aerosol absorption climatology needed in a global scaleabsorption climatology needed in a global scale
•SSA in the UV: while mean SSA at 440 nm is 0.90 (Thessaloniki) an SSA of 0.82 is needed for SSA in the UV: while mean SSA at 440 nm is 0.90 (Thessaloniki) an SSA of 0.82 is needed for eliminating GB and OMI UV differences at 305nm. SSA at UV-B wavelengths needs further eliminating GB and OMI UV differences at 305nm. SSA at UV-B wavelengths needs further investigation. investigation.
•Simple public information (e.g. UVINDEX) retrieved from OMI at such populated-urban areas Simple public information (e.g. UVINDEX) retrieved from OMI at such populated-urban areas are affected from this bias. +20% on cloudless day. are affected from this bias. +20% on cloudless day.
•Aerosol variation within an OMI satellite pixel can cause UV differences equal to a percentage Aerosol variation within an OMI satellite pixel can cause UV differences equal to a percentage (~18%) that 90% of cloudless comparison cases lie within. Statistical analysis limitations ?(~18%) that 90% of cloudless comparison cases lie within. Statistical analysis limitations ?
•Spatial and temporal UV variability has to be taken into account when comparing GB and Spatial and temporal UV variability has to be taken into account when comparing GB and satellite UV, especially at city areas.satellite UV, especially at city areas.
•Comparison under cloudy conditions requires more investigation as absolute differences are Comparison under cloudy conditions requires more investigation as absolute differences are large and spatial and temporal UV variability plays a very important role on single station – large and spatial and temporal UV variability plays a very important role on single station – satellite, comparison.satellite, comparison.
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 28
Thank you
Campaign acknowledgments:D. Balis, N. Kouremeti, V. Amiridis, M. Zebila,E. Giannakaki, J. Herman, AERONET
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 29
OMI – GB normalized biases 3 stations
27-Sep 2-Oct 7-Oct 12-Oct 17-Oct 22-Oct 27-Oct 1-Novday of Year 2007
-30
-20
-10
0
10
20
30
no
rmal
ized
OM
I - G
B %
dif
fere
nce
305 nm
380 nm
324 nm
Spatial and temporal UV variability within an OMI grid
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 30
Back up air masses 4 day back traj
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 31
Back up – Lidar 2 days
0
1
2
3
4
5
0 3 6 9 0 1 2 3 0 2 4 60
1
2
3
4
5
355 nm
BACKSC. COEF. [Mm-1sr-1]
HE
IGH
T, a
sl [
km]
06.10.2007 16.10.2007
532 nm
BACKSC. COEF. [Mm-1sr-1]
06.10.2007 16.10.2007
355 - 532 nm
COLOR INDEX
06.10.2007 16.10.2007
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 32
Back up TOMS and UVA correction
1996 1997 1998 1999 2000 2001 2002 2003 2004Year
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
To
ms
/ Bre
wer
at
324
nm
U se of additional U VA m easurem ents
No UVA m easurem ents
TOMS - BREW ER RATIO
original = 1.18 (0.13)
corrected = 1.01 (0.09)
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 33
Back up Brewer –MODIS (2000-2007)
0 0.4 0.8 1.2 1.6 2BREW ER AOD@355nm
0
0.4
0.8
1.2
1.6
2
MO
DIS
/T
err
a C
OR
RE
CT
ED
AO
D@
35
5n
m
MODI S/ Terra MEAN AOD v0.04
MODI S/ Terra MEAN AOD v0.05
MODI S/ Terra vs BREW ER 30 m inute coincidences
Equation Y = 0.8981 * X + 0.11322Num ber of data points used = 311
Average X = 0.395 & Y = 0.468R-squared = 0.576
Equation Y = 0.806 * X + 0.2059Num ber of data points used = 161
Average X = 0.353 & Y = 0.49R-squared = 0.453
21.04.23
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonths
0.00
0.20
0.40
0.60
0.80
Mea
n A
OD
at
340
nm
Clusters
#1 North W est (Atlantic)
#2 North
#3 W est
#4 East, North-East
#5 W estern, Local and Saharan dust
Mean AOD
Back up Brewer AOD (1996-2007)
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 35
Back up SSA Thessaloniki (1998-2005)
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 36
Back up SSA scout
195 196 197 198 199 200 201 202 203 204 205 206
D ay of year
0.75
0.8
0.85
0.9
0.95
1
Sin
gle
Sca
tter
ing
Alb
edo
B rew er (350nm )
C IM E L (440nm )
D U TH (550nm , average)
D U TH (550nm , average 0-1 km )
G round nephe lom eter (450nm )
21.04.23Stelios Kazadzis, OMI science team meeting Helsinki, June 2008 37
Spectral measurements of direct and global UV irradiance at the surface were made with two Brewer spectroradiometers. In addition, global (diffuse plus direct) UV irradiance and photosynthetically active radiation (PAR) were measured, on a minute basis, at each of the three sites with three NILU-UV multi-channel radiometers. In-situ measurements of aerosol vertical profiles were derived from two Lidar systems operating at (AUTH) and the site of Epanomi.Total ozone column was derived from the Brewers and cloud observations and sky images at the AUTH site. Cloud observations were performed at all sites at a half hour basis.Sun and sky radiance measurements were conducted with three CIMEL automatic sun tracking photometers, each installed at one of the three sites. These data were used to derive aerosol optical properties such as the aerosol optical depth (AOD), the Angstrom exponent a (AEa) and the single scattering albedo (SSA).
Spatial and temporal UV variability within an OMI grid